Heart failure affects 1.5% of populations, approximately three million Americans, developing at a rate of approximately 400,000 new cases per year in USA. Current therapy for heart failure is primarily directed to using angiotensin-converting enzyme (ACE) inhibitors and diuretics. ACE inhibitors appear to slow progress to end-stage heart failure; however, they are unable to relieve symptoms in more than 60% of heart failure patients and reduce mortality of heart failure only by approximately 15–20%. Heart transplantation is limited by the availability of donor hearts. With the exception of digoxin, the chronic administration of positive inotropic agents has not resulted in a useful drug without adverse side effects, including increased arrhythmias, or sudden death. These deficiencies in current therapy suggest the need for additional therapeutic approaches.
Growth of cardiac muscle cells switches from proliferation to hypertrophy during heart development. The former process is characterised by an increase in cardiac muscle cell number, and the latter by an increase in cell size without DNA synthesis or cell division. This switch is associated with terminal differentiation of cardiac muscle cells and occurs gradually during heart development, starting during the late embryonic stages and ending a few weeks after birth. During this period, gene expression, particularly that involving the cell cycle and signalling, is reprogrammed. For example, expression of a number of receptor protein tyrosine kinases and other cell cycle components decreases. Cell phenotype is also changed as cell-cell adhesions and contractile proteins are more organised in terminal differentiated myocardial cells.
Adult heart hypertrophy is an important adaptive physiological response to increased demands for cardiac work or after a variety of pathological stimuli that lead to cardiac injury. Normal hypertrophic cells have a large size with increased and well organised contractile units, as well as strong cell-cell adhesions. Although pathologically hypertrophic cells also have large size and accumulation of proteins, they often display disorganisation of contractile proteins (disarray of sarcomeric structures) and poor cell-cell adhesions (disarray of myofibers). Thus, in pathological hypertrophy, the increase in size and accumulation of contractile proteins are associated with disorganised assembly of sarcomeric structures and a lack of robust cell-cell interactions (Braunwald (1994) in Pathphysiology of Heart Failure. (Braunwald, ed.); Saunsers, Philadelphia; Vol. 14, pp 393–402).
The disarray of myofibers and sarcomeres are important features of cardiomyopathy. The former is a disorder of cell-cell association, and the latter is disorganisation of heart muscle contractile proteins. They are influenced by specific cell signals. Thus, a number of signals, like growth factors and hormones, alter cell adhesion and sarcomeric structure. Without these stimuli, cardiomyocytes display disarray of the cytoskeleton and sarcomeric structures, as well as disassociation of cell-cell interactions. As cardiac muscle cell differentiation is tightly associated with cardiac cell remodelling, adhesion and contractile protein organisations, factors that stimulate myocardial cell differentiation may be critical for enhancing the assembly of adult cardiac muscle cell sarcomeric structures.
Studies in an in vitro model system of cardiac muscle cell have led to the identification of a number of mechanical, hormonal, growth factor, and pathological stimuli which can activate several independent phenotype features of cardiac hypertrophy (Chien et al. (1991) FASEB J. 5:3037–3046; Zhou et al., (1995) PNAS. USA, 92:7391–7395). Currently, there are at least three signal transduction pathways, involving both ras-, rho- and Gq protein-dependent downstream effectors implicated in the activation of features of the hypertrophic response in these in vitro model systems. While a great deal of progress has been made in uncovering the signalling pathways which activate the ventricular muscle cell hypertrophic response, relatively little is known about the mechanisms which specifically stimulate terminal differentiation of cardiac muscle cells and the terminal differentiation-associated assembly of contractile proteins. Compounds that could influence these processes may be form a major new class of therapeutics for the treatment of a variety of cardiac diseases.
Neuregulins, a family of EGF-like growth factors, activate ErbB receptor tyrosine kinases that belong to the EGF receptor superfamily, and are involved in an array of biological responses: stimulation of breast cancer cell differentiation and secretion of milk proteins; induction of neural crest cell differentiation to Schwann cells: stimulation of skeletal muscle cell synthesis of acetylcholine receptors; and, promotion of myocardial cell survival and DNA synthesis. In vivo studies of neuregulin gene-targeted homozygous mouse embryos with severe defects in ventricular trabeculae formation and dorsal root ganglia development indicate that neuregulin is essential for heart and neural development. However, information on how neuregulin controls cell differentiation and its downstream signalling pathways is limited.
Within the heart, neuregulin and ErbB receptors are respectively expressed in the endocardial lining and cardiac muscle layer in early stages of development. Since these two layers are widely separated, the neuregulin ligand must transverse the space between the two cell layers to activate their cognate ErbB receptors. Activation of these receptors in myocardial cells is necessary for promoting muscle cell growth or migration toward the endocardium, which results in the formation of finger-like structures (ventricular trabeculae) between these two layers. It is not clear previously if neuregulin stimulates myocardial cell differentiation.
The present inventor has now found that neuregulin and/or its cellular action may be suitable for use in detection, diagnosis and treatment of heart disease. Moreover, the inventor believes that potential beneficial effects of neuregulin and/or its cellular action may be specific for heart muscle cells and not necessarily applicable to skeletal or smooth muscle cells since 1) heart, skeletal and smooth muscle are both embryological and functionally distinct; 2) factors involved in skeletal muscle growth and differentiation, such as MyoD, play little or no role in cardiac muscle growth and differentiation; 3) inactivation of the genes for ErbB2 or 4 receptors or neuregulin produces major defects in cardiac but not skeletal or smooth muscle development, 4) as shown here, the growth factor, insulin like growth factor-I (IGF-I) causes embryonic myocyte proliferation but unlike neuregulin does not stimulate differentiation of these cells. By contrast, IGF-I but not neuregulin, has been shown to induce muscle hypertrophy.